1. Field of the Invention
The present invention relates generally to active filter circuits and more specifically to an active filter circuit driven by a constant current circuit.
2. Description of the Related Art
A typical active filter circuit includes a Gm-C filter which is driven by a reference current supplied from a constant current source. Usually, the active filter circuit is formed on a common semiconductor substrate. Due to temperature drift or variability of manufacturing process, all resistors of the substrate are uniformly affected so that their resistance values deviate from their rated values. This results in the reference current varying in a direction opposite to the direction of deviation of all resistors. The effect of the varying reference current is combined with a resistance deviation that occurs in the active filter and causes a deviation of its cut-off frequency from the desired frequency.
Japanese Patent Publication 1998-284989 discloses an active filter circuit which includes a temperature-compensation current source as an extra power supply unit of main DC power source. Currents produced by the DC power source and the extra power supply unit are combined to drive a Gm-C filter. By varying the input current of the Gm-C filter according to temperature drift, the cut-off frequency of the filter is kept constant. However, the use of the extra power unit for temperature compensation requires the circuit designer to estimate all possible temperature variations and prepare reference test data based on the estimated temperature variations. The reference test data is used to adjust the output current of the extra power unit corresponding to the estimated temperature variations. While this prior art is satisfactory if the estimated temperature variations are accurate, the disclosed technique is limited for a particular type of filter. Furthermore, the prior art is incapable of compensating for the uniformly drifted variations of resistors caused by variability of manufacturing process.
Japanese Patent Publication 1995-321602 discloses a time-constant control circuit which forms part of an active filter. The time-constant control circuit is formed by a pair of transistors and a resistor coupled between the emitters of the transistors. Two current sources are respectively connected to the transistors. One of the transistors has its base biased at a reference voltage. A variability detector is provided for detecting an RC error caused by variability in the manufacturing process of integrated circuits. Based on the detected RC error, a control voltage is supplied from the variability detector to the base of the other transistor and the current sources. The time-constant control circuit produces a control voltage that renders the transconductance of the active filter unaffected by the RC error.
However, the prior art requires that the variability detector be implemented with a Gilbert multiplication circuitry which adds to the size and complexity of the integrated circuit.